How it started is the best clue
A hamstring strain causes a sudden, sharp pain at the back of the thigh during exercise, sometimes with a snap or pop when it's severe, then tenderness, swelling and bruising after a few hours, the NHS says; the American Academy of Orthopaedic Surgeons (AAOS) adds that you stop suddenly, hop or fall. Pain low in the buttock at the sitting bone that builds with running and sitting is the tendon there. Pain from the bottom down the back of the leg, often into the foot, with burning, tingling or numbness, is sciatica, and an ache in the calf, thigh or buttock that comes on walking a set distance and goes with rest can be narrowed arteries, the Circulation Foundation says. The map above points to a likely cause; an examination decides.
Sources 1, 2, 3, 4, 5
Strains are common, and they come backEmerging evidence
In 2,299 professional footballers, hamstrings made up 37% of muscle injuries, and 16% of muscle injuries were repeats. A review says nearly a third of hamstring strains come back within the first year. Older age, weak hamstrings, a previous strain and, in teenagers, growth spurts make strains more likely, NHS trust and AAOS leaflets say.
Strains are graded in different ways: by severity from 1 to 3 (an NHS trust leaflet puts these at under 10%, 11% to 49% and over 50% of the fibres torn), or on a magnetic resonance imaging (MRI) scan from 0 to 4 with a letter for where the tear is, because the old three grades say little about recovery.
Sources 6, 7, 8, 2, 9
A tendon torn off the sitting bone is easily missedEmerging evidence
The worst hamstring injury tears the tendon off the sitting bone. An editorial lists the typical features: a fall or slip that forces the hip to bend with the knee straight, a tearing or popping feeling, severe pain, sitting that hurts, difficult walking, extensive bruising down the thigh within days, and a tender sitting bone. It also lists the pitfalls: the bruising can be slight or absent, and the leg can still move fully, so it is easily mistaken for an ordinary strain. A complete tear can cause swelling and bruising down the whole leg and marked weakness, an NHS trust leaflet says.
Bruising from an ordinary thigh strain can also spread into the calf and ankle a day or 2 later, the AAOS says, so bruising alone doesn't decide it. In teenagers whose bones are still growing, the tendon can pull off a piece of bone, which shows on an X-ray.
Sources 10, 11, 2, 12
Pain at the sitting bone: the hamstring tendonEmerging evidence
Proximal (high) hamstring tendinopathy is localised pain low in the buttock with tasks such as running and sitting, and it is often long-standing. A 2016 review found no trials of rehabilitation for it; the first good trial, in 100 Australians, came in 2025 (see below). Our high hamstring tendinopathy guide covers it in full.
Sources 3, 13
Sciatica and other nerve painStrong evidence
Sciatica is pain from an irritated or squeezed sciatic nerve, felt from the bottom down the back of the leg, often into the foot, with burning, tingling, numbness or weakness, and often worse moving, sneezing or coughing, the NHS says. Estimates of how common it is range from 1.2% to 43%, because studies define it differently. A wallet in a back pocket can press on the nerves of the buttock and cause similar pain, one editorial says, and taking it out is often enough. The National Institute for Health and Care Excellence (NICE) covers how sciatica is assessed and treated.
Sources 4, 14, 15, 16
A clot can look like a strainEmerging evidence
A clot in a deep vein usually causes throbbing pain in one leg, in the calf or thigh, with swelling and red, blue or darkened skin, the NHS says. A thigh vein clot in a 24-year-old runner was first taken for a hamstring strain until a scan showed it. In one surgeon's series, 10 of 144 people with a complete hamstring tendon tear (6.9%) developed a clot, and none of 114 with a partial tear.
Sources 17, 18, 19
When a scan helpsEmerging evidence
Most strains are diagnosed from the history and examination; an ultrasound or MRI scan helps if a severe injury is suspected or recovery is slower than expected, an NHS trust leaflet says. For a possible torn tendon, the editorial's advice is to scan with ultrasound or MRI whenever the examination suggests it or leaves any doubt, and an X-ray can show a piece of bone pulled off, the AAOS says. Where the tear is matters: in a trial in sprinters, strains involving the tendon high up took more than twice as long to recover. NICE doesn't recommend routine imaging for sciatica outside specialist care, and a suspected clot is checked with a leg vein ultrasound scan.
Sources 8, 10, 2, 20, 16, 21
How it usually goesEmerging evidence
Mild hamstring strains usually get better in a few days, and severe ones can take weeks or months, the NHS says. In two trials, elite footballers returned to full training after a mean of 28 to 51 days, and sprinters and jumpers after 49 to 86 days, depending on the rehab. After a torn tendon is repaired, rehab takes at least 6 months, the AAOS says, and in a review of 387 people 76% to 100% returned to sport, though 8% to 61% had some pain left. Pain at the sitting bone improved over a year with either treatment in the 100-person trial. Sciatica usually gets better in a few weeks to a few months, but can last longer and come back. No one can promise a date.
Sources 1, 22, 20, 2, 23, 3, 4
